lo que es lo mismo, ¿cómo determinar la verdad sobre las diferentes concepciones

RFQ. Due to inelastic collisions molecules can fragment and the constituents can be investigated. First simulation studies show sufficient stopping capabilities for ions in that energy regime [Lip- pert, 2016].

Lower energetic buffer gas collisions can be realized by excitation in the injection trap, e.g. with bipolar direct current (DDC) [Webb et al., 2013] or bipolar AC applied in addition to the native RF amplitudes. The trajectories of ion species with the particular resonance frequency are enlarged with an increase of the average kinetic energy. Steady buffer gas collisions lead to inner excitation of the molecules and a rise of effective temperature. Figure 3.13 presents exemplary the rise of this temperature in dependence on the applied AC amplitude of a particular resonance amplitude (which is in the order of f_resAC≈ 1/10 fRF). The application of this method with the MR-TOF-MS

is subject of recent studies [Lippert, 2016].

Figure 3.13.: Effective temperature of ions after AC excitation versus applied resonance AC am- plitude plotted for different q-values [Lippert, 2012a].

3.4. Mass Range

The correct timing for the switching of the analyzer electrodes is essential to realize individual operation modes. So it is to realize a proper mass range. In the following, a consideration of mass range and the proper timing is given.

In an electrostatic field the time-of-flight of an ion with the general kinetic energy Ek is propor-

tional to the square root of its mass m: t ∝√m. This allows to scale the switching times for

and ejection for a given mass, in order to avoid that the ions experience any pulsed electric field. Since there is a certain time window in which a single mass is not affected by switching, one can consider this single mass as the lightest or heaviest mass in the desired mass range. This way, the minimal and maximal time can be determined.

3.4.1. Mass Range and Scaling

The following considerations are made for quadratic relation of mass and time without any ad- ditional time offset. To derive the maximal or analogously the minimal mass mmax/min (max for

maximal or min for minimal) relative to a known ’probing ion’ with mass mp(with mp_max/minas

maximal or minimal mass of the given mass range of the device), the following equation of ratios can be used: mmax/min (tmax/min)2 = mp_max/min (tp_max/min)2 (3.11) mmax/min= (tmax/min)2 (tp_max/min)2 · mp_max/min. (3.12)

Where tmax/min is the time of switching for the considered mass and tp_max/min the same for the

probe ion as the maximal or minimal mass in the spectrum. This assumption can be used to set up the timing for a desired mass and to ensure that this mass is right within the mass range. That is particularly true, whenever there is no inert time offset of the instrument t0.

The time for the the switching can be arbitrarily chosen, e.g. the time of the probing ion may be defined with the probing mass as maximal or minimal mass of the mass range - dictating the time for switching. Thereby the respectively minimal or maximal mass can be observed. Note that indices are now altered:

t_max/min:= t_{p_min/max} (3.13)

The resulting maximal or minimal mass is then given by

m_max/min=(tp_min/max) 2 (tp_max/min)2 · mp_max/min= (tp_min/max)2 (tp_max/min)2 · mp, (3.14)

where mp_max= mp_min= mp. This is true even though the ions with minimal and maximal mass

are at different positions in the analyzer at the time of switching.

The mass range from the time-of-flight point-of-view of a multiple-reflection time-of-flight mass spectrometer is given by the squared ratio of the usable time span and the total time-of-flight for the moment of the last and most restrictive switching of electrodes. One can define tin j, the time

from the injection trap to the location in the anaylzer, where the ion is influenced by the switching field

tin j=λin jtturn=λin jA

√

m/Q (3.15)

and the time tmirin the analyzer, for which the ions are influenced by the switching tmir=λmirtturn=λmirA

√

3.4. Mass Range

that are both independent on mass for fixed voltages. For Q = 1 this leads to

mmax mmin = ( N +λin j N +λin j− (1 −λmir) )2 (3.17) for the mass range of ions with equal number of turns N [Yavor et al., 2015]. With a given approximation of a completely usable range in the analyzer (λmir= 0) and a comparable time for

the injection and the turn (λin j= 1), one obtains an ideal mass ratio [Yavor et al., 2015]: mmax mmin = ( N + 1 N )2 . (3.18)

The reasonable timing to adjust the instrument can be derived analogously from this behavior of the probing ion. Again, with the assumption of t0= 0, the proper timing for any other mass can

be extracted by scaling following the ratio

m t2 = mp t2 p (3.19)

with m the desired mass, t the according time, mp the mass of the probe ion and its time tp. By

transposing t = √ m mp·t 2 p (3.20)

is obtained and can be used to get the necessary values for the particular mass m.

3.4.2. Mass Selection

A quadrupolar arrangement of electrodes in the analyzer offers selective deflection of unwanted species and can be used as Mass Range Selector (MRS) [Dickel, 2010, Toker et al., 2009]. It is primarily used to purify a spectrum and to avoid any ambiguity of turn number for different masses. Typically the mass selection for a particular time-of-flight and mass range is done during the first turns in the analyzer. This provides an unambiguous mass range (without any uncertainty of turn number) for even highest time-of-flight and reduces the amount of species (and charge) in the analyzer.

A new approach to enlarge the quasi-simultaneous mass range mrange= m_mmax_min of an MR-TOF-MS

is the scanning usage of the mass range selector. Different species can be allocated to a certain turn number by a variation of the MRS settings. This can allow for an unambiguous identification of all ions even in complex spectra. Therefore, the behavior of the MRS needs to be well-known (see section 5.1.5).

For high accurate studies of single masses, the RFQ mass filter may deliver in future aplications a pre-filtered ion beam to the trap system and hence prohibit potential space charge effects in trap and analyzer, as well as simplify the obtained mass spectra.